This invention relates, in general, to track and hold circuits, and more particularly to track and hold amplifiers for accurate high speed operation.
A track and hold amplifier is used for receiving a signal at an input stage, which occurs at some nominal frequency, and holding that signal at an output stage until that signal is received by some other circuitry. An example of the use of a track and hold amplifier is for read recovery in hard disk drives for computers.
Briefly, during track mode, an input voltage is received by an input stage and is transferred to a capacitor which is coupled to an input of an output buffer. During hold mode, the capacitor is de-coupled from the input stage thereby holding a charged voltage across the capacitor.
At least one prior art technique for holding a voltage across a capacitor has been done by coupling the capacitor to an amplifier with unity-gain feedback to hold the voltage across the capacitor during hold mode. However, a disadvantage of this technique is that it is difficult to maintain a gain of unity in the amplifier. As a result, if the feedback gain is greater than one, then the voltage across the will capacitor will drift upward. Similarly, if the feedback gain is less than one, then the voltage across the will drift downward.
Hence there exists a need for a track and hold amplifier which can operate accurately at a high input frequency, and that can hold a received voltage accurately for an output stage.